The long-term goals of this research are to gain insight into mechanisms of spinal stability and develop measurement techniques that will be critical for assessing the effects of spinal manipulative therapy (SMT) on ligament-mediated reflexes in healthy individuals and patients with low back pain. The basis of this investigation is that neuromuscular mechanisms of spinal stability are inherently connected with the structure of the spine and surrounding tissues. Therefore, techniques of SMT likely have a direct effect on mechanoreceptors in the passive tissue that may alleviate pain during movement by improving timing of stabilizing reflexes in the lumbar spine as defined by neuromuscular neutral zones (NNZs). In order to investigate the effects of manipulative therapy on NNZs, the presence and reliability of measuring these in humans must first be established using a healthy population. The mentored phase (K99) of this award will last one year, and be used to demonstrate the presence of NNZs in humans, examine which muscle are involved, and scrutinize their interactions within this phenomenon. Three hypotheses will be tested that are relevant to this single specific aim. The independent phase (ROO) will last three years, and is centered on two specific aims. The first aim will be to develop a non-invasive technique to measure intervertebral kinematics. The second aim will be to establish reliable methodology to quantify NNZs, examine differences between three rotational paradigms, and investigate sex effects on these measures. Four hypotheses will be tested relevant to these aims. This research will be conducted in a movement science laboratory located at the University of Colorado Denver. Successful completion of this research will provide foundational methodology for future testing in patient populations and implementing clinical trials designed to gain insight into therapeutic mechanisms of SMT and physiological nature of functional spinal lesions. This research trajectory will provide quantitative and evaluative assistance to clinicians when diagnosing and treating the large population of patients who seek therapy for low back pain.